1. Technical Field of the Invention
The present invention relates to heat transfer tubes having grooved inner surfaces, which are used in heat exchangers and the like in air conditioners or cooling apparatus.
2. Background Art
These types of heat transfer tubes having grooved inner surfaces are primarily used as evaporation tubes or condenser tubes in heat exchangers and the like in air conditioners or cooling apparatus. Recently, heat transfer tubes having spiraling fins formed over the entire inner surface have been widely marketed.
The heat transfer tubes which are currently most popular are manufactured by a method wherein fins are roll-formed over the entire inner surface of a metallic tube by passing a floating plug, having spiral grooves formed on the outer circumferential surface, along the interior of a seamless tube obtained by a drawing or an extrusion process. In the heat transfer tubes having outer diameters of approximately 10 mm which are commonly used, the height of the fins is about 0.15.about.0.20 mm, the pitch of the fins (the distance between the tops of adjacent fins) is about 0.45.about.0.55 mm, and the bottom width of the grooves formed between the fins is about 0.20.about.0.30 mm.
In heat transfer tubes having grooved inner surfaces with spiral fins of this type, heat transfer liquid which has collected to the bottom of the interior of the heat transfer tube is drawn up along the spiral fins by being blown by a vapor current which flows inside the tube, thereby spreading along the entire circumferential surface inside the tube. Due to this effect, the entire circumferential surface inside the tube is made almost uniformly wet, so that the area wherein boiling occurs can be increased to improve the boiling efficiency when the tube is used as an evaporation tube for vaporizing the heat transfer liquid. Additionally, when using the tube as a condenser tube for liquefying heat transfer gas, the condensation efficiency can be increased by increasing the contact efficiency between the metallic surfaces and the heat transfer gas due to the tips of the fins being exposed from the surface of the liquid.
However, it is apparent that there is room for improvement in the heat transfer efficiency due to the spiral fins. Therefore, the present inventors produced many types of heat transfer tubes having grooved inner surfaces by changing the patterns of the grooves in the heat transfer tubes, then performed experiments to compare their performance. As a result, they discovered that better heat transfer performance can be obtained in comparison to other groove patterns, if the angle of inclination of the fins formed on the inner surface of the heat transfer tubes is reciprocally changed in the circumferential direction or the axial direction.